The present invention relates to pavement markings comprising optical elements and/or skid-resistant particles. More particularly, the present invention relates to pavement markings having enhanced retroreflectivity under dry and/or wet conditions.
The use of pavement markings (e.g., paints, retroreflective elements, tapes, and raised pavement markings) to guide and direct motorists traveling along a roadway is well known. These pavement markings often are retroreflective so motorists can see the markings at night. However, when the roadway is wet, for example from rainfall, the pavement marking in turn becomes wet and often the retroreflective performance diminishes.
Retroreflection describes the mechanism where light incident on a surface is reflected so that much of the incident beam is directed back toward its source. When the surface of the pavement marking becomes wet, the optical elements (i.e., transparent, substantially spherical, glass or ceramic lenses) become coated with water, which typically reduces retroreflection. When optical elements become wetted or covered with water, the ratio of the refractive index at the exposed-lens surface changes which affects the light gathering.
To maintain good retroreflectivity during wet conditions, raised pavement markings, preformed pavement marking tapes, particularly those having raised patterned surfaces, retroreflective elements, and large diameter optical elements have been developed.
Examples of raised pavement markers include, but are not limited to, U.S. Pat. No. 4,875,798 (May et al.), U.S. Pat. No. 5,667,335 (Khieu et al.), and U.S. Pat. No. 5,667,334 (Boyce). Raised pavement markers may be used to elevate the retroreflective sheeting (i.e., raised pavement markers often comprise retroreflective sheeting (e.g., enclosed-lens, sealed-lens, or prismatic-lens sheeting)) on one or more surface(s) above any water or other liquids on the roadway. Raised pavement markings are often susceptible to scratching of the outer plastic surface. Typically, raised pavement markings are 1.3 centimeters to 3 centimeters in height. These scratches significantly reduce retroreflectivity under dry conditions. In addition, raised pavement markers are subject to damage from snowplows and often are used in combination with other forms of pavement markings to provide sufficient daytime guidance.
Preformed pavement marking tapes are generally classified as xe2x80x9cflatxe2x80x9d tapes or xe2x80x9cpatternedxe2x80x9d tapes which have vertical surfaces (typically retroreflective protuberances or protrusions (see, e.g., U.S. Pat. Nos. 4,388,359 (Ethen et al.), 4,988,555 (Hedblom), 4,988,541 (Hedblom), 5,670,227 (Hedblom et al.) and 5,676,488 (Hedblom))). Many flat pavement marking tapes rely on an exposed-lens optical system comprising transparent microspheres (i.e., optical elements) partially embedded in a binder layer containing reflective pigment particles such as titanium dioxide or lead chromate. Enclosed lens pavement marking tapes are also known (e.g., WO97/01677).
Generally, patterned pavement marking tapes have better recovery of retroreflectivity after the rain has stopped because the rain will run off the raised or vertical portions. However, water may still coat the optical elements affecting the ratio of the refractive index and thus altering (and typically decreasing) retroreflectivity.
Examples of retroreflective elements include, but are not limited to, U.S. Pat. No. 5,750,191 (Hachey et al.), U.S. Pat. No. 5,774,265 (Mathers et al.), and WO97/28470 (Palazotto et al.).
U.S. Pat. Nos. 4,072,403 (Eigenmann) and 5,268,789 (Bradshaw) describe pavement markings having good wet and dry retroreflectivity. However, the outer surface of these pavement markings may be readily scratched which decreases the dry retroreflectivity. These pavement markings tend to be rather rigid, which can make adhesion to the road difficult. Further, these pavement markings may be difficult to manufacture. The pavement markings are discreet and thus, do not provide continuous wet or dry delineation.
U.S. Pat. No. 4,145,112 (Crone) describes a wet retroreflective optical system based on refracting and retroreflective optics. One disadvantage of this system is durability. The plastic surface may scratch which reduces dry and wet retroreflective performance, particularly because this system relies on a refracting surface and on a total internal reflecting surface.
Pavement markings having a mixture of microspheres having different refractive indices have been used to obtain dry and wet retroreflectivity. See for example, U.S. Pat. No. 5,777,791 (Hedblom). Here, the higher refractive index microspheres tend to be glass which is not as durable (i.e., more readily scratched) as the lower refractive index ceramic microspheres.
EP Patent No. 385746 B1 (Kobayashi et al.) discloses a pavement marking comprising a layer of large glass microspheres embedded in the top of retroreflective enclosed-lens type base sheeting. The retroreflective pavement marking is said to be particularly useful in rainy conditions because the larger glass microspheres are partially exposed in air.
Pavement markings comprising large glass microspheres tend to recover retroreflectivity quicker after rain has stopped falling. However, actual retroreflective performance during rain tends to be poor because water covers the microsphere surface. These larger glass microspheres often have a relatively low refractive index (e.g., 1.5), which yields lower dry and wet retroreflection.
The need exists for pavement marking articles having enhanced retroreflection when wet and which provide delineation in dry and in wet conditions, and in low visibility conditions improving driver knowledge of vehicle position thereby increasing driver safety.
The present invention provides pavement marking articles which are retroreflective under dry and/or wet conditions. Surprisingly, some embodiments of the present invention have enhanced retroreflection when exposed to water, for example, when wet by rainwater. These pavement marking articles can be preformed pavement marking tapes, retroreflective flakes, or retroreflective elements embedded in a preformed pavement marking tape or in a road binder.
The articles of the present invention comprise a monolayer of exposed-lens optical elements, a spacing layer, and a reflective layer.
When the articles are a preformed pavement marking tape, the articles typically further comprise one or more top layers, a base layer, and an adhesive layer.
When the articles are retroreflective elements, the articles further comprise a core layer.
The present invention also provides a means for making these retroreflective pavement marking articles. One method comprises the steps of:
(a) providing an exposed-lens film comprising:
(i) a layer of exposed-lens optical elements;
(ii) a spacing layer; and
(iii) a reflective layer; and
(b) embossing said exposed-lens film onto a preformed pavement marking tape.
Alternatively, one or more binder materials can be applied to the exposed-lens film prior to embossing the exposed-lens film onto the preformed pavement marking tape.
The film may be selectively applied to a preformed tape. For example, the film may be applied to only the vertical surfaces, only the protrusions, in a continuous stripe down or crossweb, etc. when applied to a preformed pavement marking tape.
Alternatively, the exposed-lens film composite can be laminated to a base layer comprising a plurality of protuberances.